The present invention relates to an improvement of semiconductor devices having a high density and especially to large scale integrated circuits (an LSI) of the type in which a printed circuit board upper surface on which are defined wiring patterns superposed over the upper surface of a metal stem. End portions of lead pins extend through the superposed printed circuit board and the metal stem and are connected to corresponding wiring patterns, respectively.
In Dual Inline Packages (DIPs) which are a typical example of pin-insertion type semiconductor packages, the number of pins which can be provided is limited and in practice, is of the order of 60. It has been impossible so far to provide pins in excess of this. In order to provide the pins in excess of a order of 60, a pin insertion type such as Pin Grid Array (a PGA) in which a plurality of pins are extended from the undersurface of the package has been used in general.
Various types of PGAs have been proposed and demonstrated. One is the so-called ceramic type in which a plurality of ceramic substrates are metalized and laminated into a multilayer construction. Another is the so-called plastic type PGA in which printed wiring substrates are replaced by printed circuit board in order to achieve reductions in fabrication costs.
However, the ceramic type PGA is generally very expensive and in the case of the plastic type PGA, there arises the problem that it is generally very difficult to completely air-tightly seal the wiring substrate such as a semiconductor chip, a printed circuit board or the like so that when compared with the ceramic type PGA the reliability of operation is inferior.
In view of the above, the same inventor proposed a semiconductor device of the type which has reliable operation and which is capable of reducing the fabrication costs and can ensure complete air-tight sealing of a semiconductor chip or the like. This device is pending disclsoed in application in S.N. 095 256 and substantially solve the above and other problems encountered in the ceramic type PGA and the plastic type PGA.
The above-proposed semiconductor device will be briefly described with reference to FIGS. 1 and 2. A heat sink 2 is disposed substantially at the center portion of a metal stem 1 and a semiconductor chip 3 is mounted on the upper surface of the heat sink 2. A printed circuit board 5 whose upper surface has predetermined wiring pattern 4 extending radially and outwardly from their starting ends at the center portion around the semiconductor chip, is superposed over the upper surface of the stem 1 and the starting ends at the center portion of the wiring pattern 4 are electrically connected to their corresponding bond pads of the semiconductor chip 3 by bonding wires 6 such as aluminum wires. When the stem 1 and the printed circuit board 5 are overlaid one upon another, the upper end portion of a lead pin 7 is inserted into through holes 1a and 5a which are in alignment with each other and an insulating member 8 such as glass is filled in the annular space defined between the through hole 1a of the metal stem 1 and the lead pin 7 inserted thereinto, and over the upper surface of the printed circuit board 5, the wiring pattern 4 and the lead pin 7 are electrically interconnected with each other by a connecting member such as solder. Furthermore, the upper surfaces of the semiconductor chip 3 and the printed circuit board 5 and their sides are covered with a metal shell 10 so that the semiconductor chip 3 and the printed circuit board 5 are sealed air-tightly.
However, when the semiconductor device of the type disclosed in the above-mentioned copending U.S.S.N. 095,256 is subjected to the temperature cycle test which is defined in, for instance, MIL-STD-883C 1010.5, fatigue of the connecting member 9 such as the solder interconnecting between the upper end of lead pin 7 and the upper surface of the printed circuit board 5 occurs due to the difference in coefficient of thermal expansion between the metal stem 1 and the printed circuit board 5, and with an increase of temperature cycles, the connecting member 9 is finally damaged so that the reliable operation the semiconductor is not ensured.